1. Field of the Invention
This invention relates to a method of doping a silicon single crystal with nitrogen. More in particular, the invention relates to the addition of a nitrogen-containing powder into a melt that is used to prepare a silicon single crystal by the Czochralski method, the powder being the source of nitrogen dopant in the silicon single crystal.
2. Discussion of Related Art
The effects of nitrogen doping of silicon single crystals have been studied in the art. It is believed that silicon single crystals may be beneficially doped with nitrogen in order to enhance the formation of oxygen precipitates which act as gettering sites within the crystal, and to strengthen the crystal.
Shimura et al., "Nitrogen Effect on Oxygen Precipitation in Czochralski Silicon", Appl. Phys. Lett., vol. 48, no. 3 (Jan. 20, 1986), reported on the enhancement of oxygen precipitation in Czochralski (Cz) grown silicon wafers. It is proposed that the incorporation of nitrogen into substitutional sites generates stable microdefects in the crystal in order to enhance the oxygen precipitation. The method of doping the crystal with nitrogen is not specifically described, the reference stating only that the melt from which the silicon single crystal was grown contained 20 ppma nitrogen. How the nitrogen is introduced into the melt is not described.
Sumino et al., "Effects of Nitrogen on Dislocation Behavior and Mechanical Strength in Silicon Crystals", J. Appl. Phys., vol. 54, no. 9 (September 1983), reported that in floating-zone (FZ) grown silicon crystals, the presence of interstitial nitrogen atoms brings about a hardening of the silicon crystal through the locking of dislocations. In this regard, it is reported that nitrogen atoms thus may act in the same manner as oxygen atoms in locking crystal dislocations. The crystal was doped with nitrogen by growing the crystal in an environment of a mixed gas of argon and nitrogen.
Graf et al., "Characterization of Crystal Quality by Delineation of COP and the Impact on the Silicon Wafer Surface", Symposium on High Purity Silicon IV at the 190.sup.th Meeting of The Electrochemical Society (Oct. 6-11, 1996), reported on the size and distribution of crystal related defects in silicon prepared by the floating-zone technique. It is indicated that the number of crystal originated particles (COP) detected in the crystal decrease with nitrogen doping for a given oxygen concentration, thus indicating an involvement of oxygen in COP formation. Nitrogen doping techniques are not described.
Chen et al., J. Appl. Phys., vol. 76, no. 6 (Sep. 15, 1994), reported on the study of the formation of nitrogen-oxygen donors in nitrogen doped Cz grown silicon single crystals. The reference studied Cz silicon crystals doped with nitrogen by growing the crystals in an atmosphere of nitrogen. The background section refers to other studies of Cz silicon crystals doped with nitrogen that were formed by adding silicon nitride (Si.sub.3 N.sub.4) to the silicon melt. The form of the silicon nitride used in these other references is not described in this reference.
Abe et al., "The Characteristics of Nitrogen in Silicon Crystals", VLSI Science and Technology, W. M. Bullis and S. Broys, Eds. (1985), reported that nitrogen doping of silicon crystals showed a stronger dislocation locking compared to non-doped crystals, and that nitrogen atoms interact strongly with oxygen impurities and act as nucleation centers of oxygen precipitation. For Cz grown silicon crystals, the nitrogen was introduced by adding silicon nitride to the melt, although the specifics of the method, including the form of the silicon nitride added to the melt, are not described.
Stein, "Nitrogen in Crystalline Si", Mat. Res. Soc. Symp. Proc., vol. 59 (1986), summarizes effects of nitrogen and nitrogen-impurity interactions in crystalline silicon. It is reported that several techniques for introducing nitrogen into the silicon single crystal may be used, including nitrogen doping in the melt, forming the crystal in nitrogen gas atmospheres and nitrogen ion implantation into a formed silicon single crystal.
As noted in the references discussed above, the introduction of nitrogen into a silicon single crystal formed by the Czochralski method may be effected by several techniques, including the direct introduction of nitride materials into the melt from which the silicon single crystal is formed. As explained in U.S. Pat. No. 4,591,409, this is done by utilizing a quartz crucible lined with the nitrogen-containing material, for example, a quartz crucible lined with a silicon nitride liner. See also U.S. Pat. No. 4,090,851, describing a silicon nitride coated crucible for use in the formation of silicon single crystals. The addition of silicon wafers coated with silicon nitride to the melt may also be possible.
Known techniques for doping Cz grown silicon single crystals with nitrogen have problems, including that the methods require extra processing steps and are expensive. The preparation of silicon nitride coated crucibles and silicon nitride coated wafers involves extra processing steps which adds cost to the manufacturing process. However, these techniques are believed to be required in order to effectively dope the silicon single crystal with nitrogen in view of the fact that silicon nitride does not readily dissolve in the silicon melt. The use of nitrogen atmospheres requires special equipment and is expensive. The use of nitrogen ion implantation involves extra processing steps following formation of the silicon single crystal.
What is desired is an inexpensive method for nitrogen doping of Cz grown silicon single crystals that eliminates the need for extra processing steps.